Acquisition and Development Agreement (Details) (Mitsui E&P Texas LP, USD $) In Millions, unless otherwise specified	1 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2011
Mitsui E&P Texas LP
Acquisition and development agreement
Percentage of working interests transferred to acquiree entity (as a percent)	12.50%
Acreage of working interests transferred to acquiree entity (in acres)	39,000
Percentage of reimbursement of costs incurred on gathering assets at time of closing (as a percent)	50.00%
Percentage of the entity's costs and expenses during the first three years following the closing of the transaction, borne by the acquiree entity (as a percent)		90.00%
Costs and expenses incurred by acquiree entity on behalf of entity that the acquiree entity has agreed to pay		$ 680.0